(A) Representative histological paraffin sections of Matrigel plugs. In CEACAM1^endo+ mice, newly formed vessels in the plug area were CEACAM1 positive (anti-CEACAM1 antiserum 2456, blue; counterstain, Nuclear Fast Red) and PECAM1-positive (red; counterstain, Mayer’s hemalaun). Plugs shown here contained recombinant Ang-1. In plugs from Ceacam1^–/– mice (counterstain, Mayer’s hemalaun), the majority of CEACAM1-negative vessel-like structures in Matrigel plugs were defective: extravascular erythrocytes were observed (arrows). This area is enlarged at right. Implants from the corresponding WT siblings of CEACAM1^endo+ and Ceacam1^–/– mice are also shown. Newly formed vessels revealed reactivity toward CEACAM1 and PECAM1 antibodies. Magnification, ×200, ×400 (inset). (B) Immunofluorescent labeling of CEACAM1 and PECAM1 in newly formed capillaries in a Matrigel plug from a CEACAM1^endo+ mouse. Congruent endothelial expression of CEACAM1 (green, anti-CEACAM1 antiserum 2456) and PECAM1 (red, anti-PECAM1 antibody BM4086) is shown in the overlay. Magnification, ×200. (C) Neovascularization in Matrigel plugs from CEACAM1^endo+ mice and their FVB/N WT siblings. After angiogenic challenge with Ang-1, bFGF, and VEGF, PECAM1-positive vessels were counted per field. A total of 3 fields from individual plugs of 3 animals per group were counted. (D) Quantitative comparison of vascularization in Matrigel implants from Ceacam1^–/– mice and their WT littermates. In Ceacam1^–/– mice, vessel-like structures associated with erythrocytes were also considered. Significant differences in the number of vessels formed in Matrigel implants in CEACAM1^endo+ and Ceacam1^–/– mice compared with their WT littermates are indicated by asterisks (*P < 0.05).